Fix migration pull request title too long (go-gitea#34577)

Fix go-gitea#34294

Author: lunny

services/migrations/gitea_uploader.go

@@ -765,7 +765,7 @@ func (g *GiteaLocalUploader) newPullRequest(ctx context.Context, pr *base.PullRe
 	issue := issues_model.Issue{
 		RepoID:      g.repo.ID,
 		Repo:        g.repo,
-		Title:       prTitle,
+		Title:       util.TruncateRunes(prTitle, 255),
 		Index:       pr.Number,
 		Content:     pr.Content,
 		MilestoneID: milestoneID,